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Network Working Group J. Snell Request for Comments: 4685 September 2006 Category: Standards Track

                     Atom Threading Extensions

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).


 This memo presents a mechanism that allows feeds publishers to
 express threaded discussions within the Atom Syndication Format.

Table of Contents

 1. Introduction ....................................................1
 2. Notational Conventions ..........................................2
 3. The 'in-reply-to' Extension Element .............................2
 4. The 'replies' Link Relation .....................................5
 5. The 'total' Extension Element ...................................6
 6. Considerations for Using thr:count, thr:updated, and total ......7
 7. Security Considerations .........................................8
 8. IANA Considerations .............................................9
 9. References ......................................................9
    9.1. Normative References .......................................9
    9.2. Informative References ....................................10
 Appendix A.  Acknowledgements .....................................11

1. Introduction

 This document defines an extension for expressing threaded
 discussions within the Atom Syndication Format [RFC4287].

Snell Standards Track [Page 1] RFC 4685 Feed Thread September 2006

2. Notational Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in BCP 14, [RFC2119], as
 scoped to those conformance targets.
 The XML Namespaces URI [W3C.REC-xml-names-19990114] for the XML
 elements and attributes described in this specification is:
 In this document, the namespace prefix "thr:" is used for the above
 Namespace URI.  Note that the choice of namespace prefix is arbitrary
 and not semantically significant.
 This specification uses a shorthand form of terms from the XML
 Infoset [W3C.REC-xml-infoset-20040204].  The phrase "Information
 Item" is omitted when naming Element and Attribute Information Items.
 Therefore, when this specification uses the term "element," it is
 referring to an Element Information Item in Infoset terms.  Likewise,
 when this specification uses the term "attribute," it is referring to
 an Attribute Information Item.
 This specification allows the use of IRIs [RFC3987].  Every URI
 [RFC3986] is also an IRI, so a URI may be used wherever an IRI is
 named.  When an IRI that is not also a URI is given for
 dereferencing, it MUST be mapped to a URI using the steps in Section
 3.1 of [RFC3987].  When an IRI is serving as an identifier, it MUST
 NOT be so mapped.
 Some sections of this specification are illustrated with a non-
 normative RELAX NG Compact schema [RELAXNG].  In those sections, this
 specification uses the atomCommonAttributes, atomMediaType, and
 atomURI patterns, defined in [RFC4287].
 However, the text of this specification provides the sole definition
 of conformance.

3. The 'in-reply-to' Extension Element

 The "in-reply-to" element is used to indicate that an entry is a
 response to another resource.  The element MUST contain a "ref"
 attribute identifying the resource that is being responded to.
 The element is not unlike the references and in-reply-to email
 message headers, defined by [RFC2822].  However, unlike the in-
 reply-to header, the "in-reply-to" element is required to identify
 the unique identifier of only a single parent resource.  If the entry

Snell Standards Track [Page 2] RFC 4685 Feed Thread September 2006

 is a response to multiple resources, additional "in-reply-to"
 elements MAY be used.  There is no direct equivalent to the
 references header, which lists the unique identifiers of each
 preceding message in a thread.
 in-reply-to =
   element thr:in-reply-to {
     ( undefinedContent )
 ref = attribute ref { atomURI }
 href = attribute href { atomURI }
 type = attribute type { atomMediaType }
 source = attribute source { atomURI }
 The "ref" attribute specifies the persistent, universally unique
 identifier of the resource being responded to.  The value MUST
 conform to the same construction and comparison rules as the value of
 the atom:id element, as defined in Section 4.2.6 of [RFC4287].
 Though the IRI might use a dereferenceable scheme, processors MUST
 NOT assume that it can be dereferenced.
 If the resource being responded to does not have a persistent,
 universally unique identifier, the publisher MUST assign an
 identifier that satisfies all the considerations in Section 4.2.6 of
 [RFC4287] for use as the value of the "ref" attribute.  In that case,
 if a representation of the resource can be retrieved from an IRI that
 can be used as a valid atom:id value, then this IRI SHOULD be used as
 the value of both the "ref" and "href" attributes.
 The "source" attribute MAY be used to specify the IRI [RFC3987] of an
 Atom Feed or Entry Document containing an atom:entry with an atom:id
 value equal to the value of the "ref" attribute.  The IRI specified,
 once appropriately mapped to a corresponding URI, MUST be
 The "href" attribute specifies an IRI that may be used to retrieve a
 representation of the resource being responded to.  The IRI
 specified, once appropriately mapped to a corresponding URI, MUST be

Snell Standards Track [Page 3] RFC 4685 Feed Thread September 2006

 The "type" attribute MAY be used to provide a hint to the client
 about the media type [RFC4288] of the resource identified by the
 "href" attribute.  The "type" attribute is only meaningful if a
 corresponding "href" attribute is also provided.
 This specification assigns no significance to the order in which
 multiple "in-reply-to" elements appear within an entry.
 An example of an entry with a response follows:
 <feed xmlns=""
   <title>My Example Feed</title>
   <link href="" />
     <title>My original entry</title>
       href="" />
     <summary>This is my original entry</summary>
     <title>A response to the original</title>
     <link href="" />
     <summary>This is a response to the original entry</summary>
 To allow Atom processors that are not familiar with the in-reply-to
 extension to know that a relationship exists between the entry and
 the resource being responded to, publishers are advised to consider
 including a "related" link referencing a representation of the
 resource identified by the in-reply-to element.  Although such links
 are unlikely to be processed as a reference to a predecessor in a
 threaded conversation, they are helpful in at least establishing a
 semantically meaningful relationship between the linked resources.

Snell Standards Track [Page 4] RFC 4685 Feed Thread September 2006

 For example,
 <feed xmlns=""
   <title>My Example Feed</title>
   <link href="" />
     <title>A response to the original</title>
     <link href="" />
       source="" />
       href="" />
     <summary>This is a response to the original entry</summary>

4. The 'replies' Link Relation

 An Atom link element with a rel attribute value of "replies" may be
 used to reference a resource where responses to an entry may be
 found.  If the type attribute of the atom:link is omitted, its value
 is assumed to be "application/atom+xml".
 A "replies" link appearing as a child of the Atom feed or source
 element indicates that the referenced resource likely contains
 responses to any of that feed's entries.  A "replies" link appearing
 as a child of an Atom entry element indicates that the linked
 resource likely contains responses specific to that entry.
 An atom:link element using the "replies" rel attribute value MAY
 contain a "thr:count" attribute whose value is an unsigned, non-
 negative integer, conforming to the canonical representation of the
 XML Schema nonNegativeInteger data type [W3C.REC-xmlschema-2-
 20041028], that provides a hint to clients as to the total number of
 replies contained by the linked resource.  The value is advisory and
 may not accurately reflect the actual number of replies.

Snell Standards Track [Page 5] RFC 4685 Feed Thread September 2006

 The link MAY also contain a "thr:updated" attribute, whose value is a
 [RFC3339] date-time stamp conforming to the same construction rules
 as the Atom Date Construct defined in [RFC4287], and is used to
 provide a hint to clients as to the date and time of the most
 recently updated reply contained by the linked resource.  The value
 is advisory and may not accurately reflect the actual date and time
 of the most recent reply.
 For example,
 <feed xmlns=""
   <title>My Example Feed</title>
   <link href="" />
     <title>My original entry</title>
     <link href="" />
     <link rel="replies"
           thr:count="10" thr:updated="2005-07-28T12:10:00Z" />
     <summary>This is my original entry</summary>
 Although Atom feed, entry, and source elements MAY each contain any
 number of atom:link elements using the "replies" link relation, this
 specification assigns no significance to the presence or order of
 such links.  Multiple replies links appearing within an atom:entry
 may reference alternative representations of the same set of
 responses or may reference entirely distinct resources containing
 distinct sets of responses.  Processors MUST NOT assume that multiple
 replies links are referencing different representations of the same
 resource and MUST process each replies link independently of any

5. The 'total' Extension Element

 The "total" element is used to indicate the total number of unique
 responses to an entry known to the publisher.  Its content MUST be an
 unsigned non-negative integer value conforming to the canonical
 representation of the XML Schema nonNegativeInteger data type

Snell Standards Track [Page 6] RFC 4685 Feed Thread September 2006

    total = element thr:total { xsd:nonNegativeInteger }
 Atom entries MAY contain a "total" element but MUST NOT contain more
 than one.
 There is no implied relationship between the value of the "total"
 element of an Atom entry and any individual or aggregate values of
 the "thr:count" attributes of its Atom link elements having a
 "replies" relation.

6. Considerations for Using thr:count, thr:updated, and total

 The thr:count, thr:updated, and total extensions provide additional
 metadata about the thread of discussion associated with an entry.
 The values are intended to make it easier for feed consumers to
 display basic contextual information about the thread without
 requiring that those consumers dereference, parse, and analyze linked
 resources.  That said, there are a number of considerations
 implementors need to be aware of.
 First, these extensions MUST NOT be assumed to provide completely
 accurate information about the thread of discussion.  For instance,
 the actual total number of responses contained by a linked resource
 MAY differ from the number specified in the thr:count attribute.
 Feed publishers SHOULD make an effort to ensure that the values are
 accurate.  The non-authoritative nature of "external reference
 metadata", like the replies link attributes, is discussed in detail
 in Section 3.3 of the W3C document "Tag Finding 12:  Authoritative
 Metadata" [TAG12].
 Second, the values of the these extensions are volatile and may
 change at a faster rate than that of the containing entry.  Frequent
 updates to these values, or to any part of the Atom document, could
 have a detrimental impact on the cacheability of the document using
 the attributes, leading to an increase in overall bandwidth
 Feed publishers SHOULD consider a change to the values of the thr:
 count, thr:updated, and total extensions an "insignificant" update in
 terms of [RFC4287], meaning that the value of the containing feed,
 entry, or source element's atom:updated element SHOULD NOT be
 affected by a change to the values of these extensions.
 Lastly, implementors need to be aware that although the Atom
 specification [RFC4287] explicitly allows the link element to contain
 arbitrary extensions, the specification does not require that
 implementations support such extensions.  Specifically, relating to
 the use of extensions, Atom does not define any level of mandatory

Snell Standards Track [Page 7] RFC 4685 Feed Thread September 2006

 conformance on the part of feed consumers beyond a requirement that
 implementations ignore any extension the implementation does not
 understand.  As a result, some implementations MAY NOT be capable of
 fully utilizing the extensions defined by this or any specification.

7. Security Considerations

 As this specification defines an extension to the Atom Syndication
 Format, it is subject to the same security considerations defined in
 Feeds using the mechanisms described here could be crafted in such a
 way as to cause a consumer to initiate excessive (or even an unending
 sequence of) network requests, causing denial of service (to the
 consumer, the target server, and/or intervening networks).  Consumers
 can mitigate this risk by requiring user intervention after a certain
 number of requests, or by limiting requests either according to a
 hard limit, or with heuristics.
 The mechanisms described here can be used to construct threaded
 conversations spanning resources distributed across multiple domains.
 For example, an individual posting an entry to one weblog hosted on
 one Internet domain could mark that entry as a response to an entry
 from a different weblog hosted on a different domain.  Implementors
 should note that such distributed responses can be leveraged by an
 attacker to attach inappropriate or unwanted content to a discussion.
 Such attacks can be prevented or mitigated by allowing users to
 explicitly configure the sources from which responses may be
 retrieved, or by applying heuristics to determine the legitimacy of a
 given response source.
 Implementors should also note the potential for abuse that exists
 when malicious content publishers edit or change previously published
 content.  In closed, centralized comment systems, after-the-fact
 editing of comments is typically not an issue, as such changes are
 easily prevented, detected, or tracked.  With the form of distributed
 comments enabled through the use of the thr:in-reply-to extension,
 however, such changes become more difficult to detect, raising the
 possibility of serious attribution and repudiation concerns.  XML
 Digital Signatures, as specified in Section 5.1 of [RFC4287], present
 one possible avenue for mitigating such concerns, although the
 presence of a valid XML Digital Signature within an entry is not, by
 itself, a reliable defense against repudiation issues.

Snell Standards Track [Page 8] RFC 4685 Feed Thread September 2006

8. IANA Considerations

 This specification defines one new Atom link relation type that has
 been registered in the IANA Registry of Link Relation, as defined by
    Attribute Value: replies
    Description: (see Section 4)
    Expected display characteristics: (see Section 4)
    Security considerations: (see Section 5)

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
            Timestamps", RFC 3339, July 2002.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
            Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC4287]  Nottingham, M. and R. Sayre, "The Atom Syndication
            Format", RFC 4287, December 2005.
 [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
            Registration Procedures", BCP 13, RFC 4288, December 2005.
            Tobin, R. and J. Cowan, "XML Information Set (Second
            Edition)", W3C REC REC-xml-infoset-20040204, February
            Hollander, D., Bray, T., and A. Layman, "Namespaces in
            XML", W3C REC REC-xml-names-19990114, January 1999.
            Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
            Second Edition", W3C REC REC-xmlschema-2-20041028, October

Snell Standards Track [Page 9] RFC 4685 Feed Thread September 2006

9.2. Informative References

 [RELAXNG]  Clark, J., "RELAX NG Compact Syntax", December 2001,
 [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
 [TAG12]    Fielding, R. and I. Jacobs, "Tag Finding 12: Authoritative
            Metadata", <

Snell Standards Track [Page 10] RFC 4685 Feed Thread September 2006

Appendix A. Acknowledgements

 The author gratefully acknowledges the feedback from Antone Roundy,
 Aristotle Pagaltzis, Byrne Reese, David Powell, Eric Scheid, James
 Holderness, John Panzer, Lisa Dusseault, M. David Peterson, Sam Ruby,
 Sylvain Hellegouarch, and the remaining members of the Atom
 Publishing Format and Protocol working group during the development
 of this specification.  Any fault or weakness in the definition of
 this extension is solely the blame of the author.
 Some portions of text in this document have been adapted from
 [RFC4287] in order to maintain a stylistic and technical alignment
 with that specification.

Author's Address

 James M Snell

Snell Standards Track [Page 11] RFC 4685 Feed Thread September 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

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Snell Standards Track [Page 12]

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